1. Field of the Invention
The present invention relates to a linear image sensor capable of optically reading an image on a document, and more particularly to a long size integrated image sensor which is capable of reading an isometric image formed on the sensor surface and used in a document-reading scanner, a facsimile, etc.
2. Description of the Prior Art
Conventionally, image sensors formed on a single-silicon crystal substrate by applying IC technology are known as CCD image sensors and MOS image sensors for example. Additionally, image sensors formed on either a glass or quartz substrate by applying thin-film technology are known as CdS-CdSe image sensors and amorphous Si image sensors, for example.
Generally, any conventional image sensor is comprised of a plurality of photosensors and a scanning circuit. The CCD (charge-coupled device) image sensor incorporates photodiodes as the photosensors and a CCD as the scanning circuit. The MOS image sensor incorporates photodiodes as the photosensors and a shift register comprised of MOS transistors as the scanning circuit. The CdS-CdSe image sensor incorporates photoconductors made from CdS-CdSe film as the photosensors and an x-y matrix circuit as the scanning circuit. The amorphous Si image sensor incorporates either photodiodes or photoconductors made from amorphous silicon as the photosensors and either a shift register or an x-y matrix circuit made from amorphous silicon as the scanning circuit.
From the view point of an image reading method, there are two types of image reading apparatus: one that reads a reduced image formed on the sensor surface; and the other that reads an isometric image of the ducument formed on the sensor surface. The reduced image reading type has already been introduced to facsimile equipment. However, it has limitations in reduction of size, resolution and sensitivity. Conversely, this type uses a sensor having a short effective sensing length , thus saving cost of the sensor itself. The isometric image reading type is compact in size due to short optical length. It also has a large photosensing area, high sensitivity, and easy adjustability of the optical system. Conversely, this type needs a sensor having a length nearly equal to the full width of the document to be read, thus eventually resulting in high sensor cost. Since the isometric image reading type has a relatively short (20 through 50 mm) optical path, the image sensor used for this type is called "contact-type image sensor".
Recently, reflecting the above advantages of the isometric image reading method, a variety of contact-type image sensors have been developed. The contact-type image sensor uses sensor elements arranged at relatively wide pitches, (e.g., 62.5 .mu.m pitch for 16 dots/mm resolution, and 125 .mu.m pitch for 8 dots/mm resolution). The contact-type image sensor, having the configuration mentioned above, include the types having a CCD contact-type image sensor which incorporates a plurality of CCD image sensors disposed either linearly or zigzag on a long substrate, and CdS-CdSe and amorphous image sensors which are also integrated on a long substrate. Since the CCD image sensor uses charge carriers as signal carriers which are not at thermally equilibrium, dark current increases at a high temperature range, so that the CCD image sensor cannot easily be used at high temperature range. On the other hand, when reading a reduced image, due to an extremely small size (7 through 14 .mu.m) of the potential well of the CCD, the reduced image can be scanned at an extremely fast speed. Conversely, when reading an isometric image, a greater size (62.5 through 125 .mu.m) of potential well is needed, thus resulting in slower image scanning speed, greater area of charge-transfer gate, increased capacity of the transfer clock signal lines, and larger AC power consumption. Using a long CCD image sensor having sensor chips being disposed zigzag on a long substrate requires rearranging the image signals after reading the document, so that, complex driver circuit and output-signal processing circuit are unavoidably needed.
On the other hand, since the CdS-CdSe image sensor utilizes a photoconductive effect of the photosensing elements, it provides a slow photoresponsive speed (at fastest 5 mS), so that it is not suited for reading an image at a high speed. Likewise, an amorphous Si image sensor utilizing the photoconductive effect of the photosensing elements is not suited for reading image at a high speed. Another amorphous Si image sensor using photodiodes for making up photosensing elements provides faster photoresponsive speeds. However, due to the insufficient sensitivity and S/N (signal-to-noise) ratio, this is also not suited for reading an image at high speed and high quality. A scanning circuit, i.e., x-y matrix circuit or shift register made from thin-film elements, is inferior to that made from Si crystal elements in operation speed and read image quality. Taking this into account, image sensors made from thin-film elements are mostly applied to low-speed and inexpensive image sensors due to its advantage for allowing integral production of long size, rather than aiming at such performances of operation speed, resolution and S/N ratio.